1. Field of the Invention
The present invention relates to a sputtering device used in the fabrication of various types of semiconductor articles. More particularly, it relates to a sputtering device equipped to ionize sputter particles.
2. Related Art
Various types of memory and logic devices have wiring patterns and barrier films formed by sputtering thin film coatings on a surface of a semiconductor substrate material. There has recently been a great need for efficient coating of the inner peripheral surfaces of holes formed in such substrates.
For example, a CMOS-FET (field effect transistor), which is commonly used in a DRAM, employs a structure that prevents cross-contamination between the contact wiring layer and the diffusion layer by providing a barrier film on the inner surfaces of contact holes provided in the diffusion layer.
In another example, through holes are provided in a substrate which is used for the wiring of memory cells. A multilayer wiring structure is embedded inside these through holes in order to link a lower wiring layer with an upper wiring layer. These through holes also lead to an interlayer insulation film. Here again, a structure is adopted in which a barrier film is produced inside the through holes to prevent cross-contamination of the various wiring and insulating patterns.
Due to the increasing demand for integration in semiconductor devices, the ratio of the hole depth to the size of the hole opening (hereafter referred to as the "aspect ratio") has been steadily rising over the years. For example, a typical aspect ratio of holes in a 64 megabit DRAM device is about 4, while a typical aspect ratio of holes in a 256 megabit DRAM is about 5 to 6.
In the case of a barrier film, a thin film must be built up on the bottom of the hole. For holes with a high aspect ratio, it is difficult to deposit a film of sufficient thickness on the bottom of the hole. As shown in FIG. 2a, when a thin film 510 is deposited inside a fine hole 500 formed in the surface of the substrate 50, there is a tendency for the thin film 510 to build up and create a bulge around the edge 503 of the opening of the hole 500. The thin film 510 that makes up this bulge portion is called an "overhang. " The formation of this overhang raises the apparent aspect ratio through a narrowing of the opening to the hole 500. The amount of sputter atoms that reach the inside of the hole 500 decreases, and the bottom coverage is diminished.
A decrease in the bottom coverage can lead to a thinner barrier film at the bottom of the hole and to critical flaws in the device characteristics, such as junction leakage. Typically, the rate at which the bottom of the hole is coated, i.e., the thickness of deposited coating per unit of time should be around 10% to 15% of the rate at which the peripheral surface of the hole is coated in order to provide a bottom coating of sufficient thickness.
Collimation sputtering and low-pressure, long-distance sputtering processes have been utilized to increase the bottom coverage.
Collimation sputtering involves using a plate (collimator) in which numerous holes have been made in the direction perpendicular to the substrate, and providing this plate between the target and the substrate. Collimation sputtering is a process in which only those sputter particles (usually sputter atoms) that fly more or less perpendicular to the substrate are selectively allowed to reach the substrate. A problem with collimation sputtering is that sputter particles accumulate on the collimator portion, and the resulting loss of material decreases the deposition rate of the sputter particles. Because of these problems, collimation sputtering is only used for mass-produced products of the 16-megabit class in which the aspect ratio is up to about 3.
Low-pressure, long-distance sputtering involves lengthening the distance between the target and substrate (usually about 3 to 5 times farther) so that the sputter particles that fly more or less perpendicular to the substrate are the particles that are most likely to land on the substrate. Lowering the pressure more than usual (about 0.8 mTorr or less) has the effect of increasing the mean free path of the sputter particles. The mean free path is generally defined as the average distance the particles travel before they collide with other particles or ions. By reducing pressure, turbulence of the sputter particles is reduced and the mean free path increases, resulting in more sputter particles reaching the substrate. A problem with low-pressure, long-distance sputtering is that since the pressure is lowered and the distance between the target and the substrate is lengthened, there is a fundamental decrease in the deposition rate. Therefore, long-distance sputtering is limited to devices up to an aspect ratio up to about 4.